Magnetic head

ABSTRACT

A multitrack magnetic head assembly is described in which magnetic ferrite pole tips are held in place at the transducing face of the assembly by a ceramic material to which the ferrite pole pieces are glass bonded. This face part is in turn secured to a core housing carrying a plurality of core members and coil means disposed to complete the magnetic circuit through the ferrite pole pieces. As magnetic ferrite materials have many characteristics similar to the ceramic material, the face part composed of these two substances becomes a structure of high physical integrity capable of lasting under the abusive conditions of high magnetic tape speeds. Also, a method of constructing this head assembly is provided in which the ceramic face part is comprised of two half portions each carrying a set of ferrite pole pieces glass bonded therein such that the ceramic half portions can be assembled with a layer of nonmagnetic material disposed therebetween to form the transducing gaps between each associated pair of ferrite pole pieces. Once the face part portions are assembled and secured, this sub-assembly is mounted on the core housing, also of ceramic material, in such a way that the core members are arranged in the core housing to engage the ferrite pole pieces.

[Jnited States Patent [191 Kroon [4 Oct. 22, 1974 MAGNETIC HEAD [75]lnventor: William L. Kroon, Sunnyvale, Calif.

[73] Assignee: Ampex Corporation, Redwood City,

Calif.

22 Filed: 01:1.24, 197 3 [21] Appl. No.: 409,045

Related US. Application Data [63] Continuation of Ser. No, 156,801, June25, 1971,

abandoned.

[52] US. Cl 360/121, 360/123, 360/125 [51] Int. Cl Gllb 5/22, G1 lb5/28, G1 lb 5/20 [58] Field of Search 179/1002 C; 340/174.1 F; 346/74MC; 360/121, 123, 125

Primary Examiner-Alfred H. Eddleman [5 7] ABSTRACT A multitrack magnetichead assembly is described in which magnetic ferrite pole tips are heldin place at the transducing face of the assembly by a ceramic materialto which the ferrite pole pieces are glass bonded. This face part is inturn secured to a core housing carrying a plurality of core members andcoil means disposed to complete the magnetic circuit through the ferritepole pieces, As magnetic ferrite materials have many characteristicssimilar to the ceramic material, the face part composed of these twosubstances becomes a structure of high physical integrity capable oflasting under the abusive conditions of high magnetic tape speeds. Also,a method of constructing this head assembly is provided in which theceramic face part is comprised of two half portions each carrying a setof ferrite pole pieces glass bonded therein such that the ceramic halfportions can be assembled with a layer of nonmagnetic material disposedtherebetween to form the transducing gaps between each associated pairof ferrite pole pieces. Once the face part portions are assembled andsecured, this sub-assembly is mounted on the core housing, also ofceramic material, in such a way that the core members are arranged inthe core housing to engage the ferrite pole pieces.

6 Claims, 26 Drawing Figures PNEWEDUU 22 m4 INVENTOR.

WILLlAM L. KROON ATTORNEY 4 PATENTEmm 22 m4 INVENTOR.

WILLIAM L. KROON TTORNEY smam PATENTEMU 22 saw:

' INVENTDR.

'wu HAM l hiHOv-p;

ATTORNEY MAGNETIC HEAD This is a continuation of Applicants priorcopending application Ser. No. 156,80I, filed June 25, 1971, nowabandoned.

In general, the present invention relates to magnetic transducers andmore particularly to a multitrack head assembly employing transducercomponents of magnetic ferrite material and to a method of constructingsuch a head assembly.

Most all multitrack magnetic heads in use today are formed with magnetictransducer poles of metallic material. In certain applications, metalssuch as Alfenol, Silconol, and Alfesil are preferred due to theirrelatively high permeability and physical hardness, while otherapplications call for the softer but higher permeability of laminatedpermalloy or mumetal transducer parts. Heads constructed of any of thesematerials are subject to abbreviated lives due to the abrasive'wearingeffect of tape, particularly in high performance instrumentation anddigital transports where tape speeds range from 50 to 120 inches persecond. It was early thought that the solution to the lack of a durablematei rial having good magnetic properties could be found in the use ofmagnetic ferrites. Ferrites are characteristically very hard and exhibitan adequate permeability at low frequencies and a very desirable highfrequency permeability. Unfortunately, by reason of the extreme hardnessof ferrites, this material is also very brittle and a poor thermalconductor rendering it troublesome to maintain a good definition of thenonmagnetic gap which is formed between two ferrite pole pieces. Also,many practical difficulties are encountered in constructing a magnetichead of this material having mechanically desirable characteristics.These latter problems concern the workability and the internal strengthof ferrites and are particularly significantin the construction of highperformance instrumentation and digital recorders wherein the trackingazimuth and gap scatter of the multitrack head must be held to extremelyfine tolerances. For example, it might be thought a simple matter tomount the ferrite pole pieces in an open face housing and secure thetransducer components in place by potting with an epoxy. This type ofconstructiomhowever, has been found unsatisfactory due to theconsiderable heat developed at the head face due to the abrasive effectsof the moving tape and/or environmental conditions which cause the epoxymaterial to creep or otherwise release the transducer components fromtheir original positions giving rise to non-zero track azimuth and gapscatter. It is noted that the difficulty with using a potting or epoxycompound at the head face has also been demonstrated when usingnonferrite metal transducers. In fact, in the case of transducers formedof metallic Alfenol or Silconol type materials, the industry hasresorted to a head structure consisting of a metal face where the activemagnetic pole pieces are surrounded, except for microscopic gaps, by auniform metal surface against which the tape bears. Such a metal housingrigidly maintains the metal pole pieces in the desired orientation. Suchall metal heads have in fact been the most successful high performancemagnetic'heads at this time.

Accordingly, it is an object of the present invention to provide notonly animproved long life high performance multichannel magnetic headbut also to provide a method for constructing such a head adapted tolarge scale production.

These and other objects, features and advantages of the invention willbecome apparent from the following description and accompanying drawingsdescribing and illustrating the preferred embodiment of the invention,

' wherein:

FIG. 1 is a front elevation view of the magnetic head assembly partiallycut away for clarity;

FIG. 2 is a side elevation view also partially cut away to display theinternal components;

FIG. 3 is a section view of the head assembly of FIG.

1 taken along lines 33 thereof; I

FIG. 4 is a perspective view of the magnetic head assembly;

FIGS. 5A through 5L illustrate the various steps involved in the methodof constructing the magnetic head assembly in accordance with thepresent invention; and

FIGS. 6A through 61 illustrate the various steps for constructing analternative preferred embodiment of the head assembly in accordance withthe invention.

With reference to FIGS. 1-4, the magnetic head assembly of the presentinvention comprises a head face part 11 of a ceramic material in which aplurality of separate magnetic pole pieces 12 and 13 are inserted. Inaccordance with the invention, each of the plurality of pole pieces 12and 13 are of a magnetic ferrite material and are glass bonded to thesurfaces of part II in contact therewith. According to the method of thepresent invention, face part 11 is in fact formed in two half portions11a and 11b, each carrying a half set of the pole pieces such that inthe completed head assembly, part portions 11a and 11b and the pluralityof pole tips 12 and 13 abut at a gap region 14 in which a layer ofnonmagnetic material is disposed to define the transducing gaps. Facepart 11 with the various pole pieces 12 and 13 mounted therein isfabricated as a unit and is thereupon mounted onto a core housing 16,preferably also of a ceramic material, carrying a plurality of magneticcore members 17, each provided with an electrical coil 18. Each coremember 17 is arranged to engage associated pairs of pole pieces 12 and13 of face part 11 so as to complete a magnetic circuit in each casetraversing the nonmagnetic transducing gap at region'l4. Thus, separatebranches 21 and 22 of each core member 17 are positioned so as to engageassociated pole pieces 12 and 13 on opposite sides of the nonmagneticgap. In this manner, a plurality of transducers are formed, eachcomprising a pair of pole pieces carried in front face 11 and a singleintegral core member mounted within housing 16.

A principal feature of the present invention resides in the ceramiccomposition of face part 11 in combination with the ferrite pole pieces12 and 13. As the ceramic material exhibits extreme hardness, similar tothe magnetic ferrite material, it provides an extremely low rate of wearwhen subjected to the abrasive effects of magnetic tape driven at highspeeds. Furthermore, unlike magnetic heads formed with an epoxy orplastic face, the ceramic face part firmly retains the ferrite polepieces against any physical displacement, even under operatingconditions where the head is subjected to substantial heat, therebyassuring negligible individual track azimuth and gap scatter. A furtheradvantage realized by this construction, is that the ceramic materialmay be selected to have a coefficient of thermal expansion within thesame range as the magnetic ferrite material used for the pole pieces 12and 13. Under operating conditions, the face portion of the headassembly may be subjected to a wide range of temperatures such as causedby the frictional effects of the moving tape or environmentalconditions. However, as the expansion and contraction of face part 11and that of the ferrite pole pieces are substantially the same, there isminimal stress placed on the ferrite parts and the critical gap regiontherebetween, thereby giving further stability to the physical integrityof the structure. Additionally, the glass seals formed at the face partprevent absorption of moisture into the assembly which might otherwisecause damage.

As described more fully herein, in the preferred method of constructingthis head assembly, each of the ferrite pole pieces 12 and 13 are glassbonded to the surrounding ceramic material of face part 11. The glassmaterial employed as a flux for this purpose, is like the ceramicmaterial comprising face part 11, selected to have a coefficient ofthermal expansion within the same range as that of the magnetic ferritematerial. Thus, the entire face part sub-assembly is formed into adurable ceramic-glass-ferrite unit, providing an extremely hard surfacecapable of being polished to a high degree so as to afford the leastpossible friction to the magnetic tape.

In order to define the depth (vertical direction in FIG. 3) oftransducing gaps 23 when the head is finished, face part 11 and polepieces 12 and 13 are provided with a V-shaped groove 24 extendingparallel and beneath gap region 14 with the groove apex defining thebottom of nonmagnetic gap 23 between each pair of pole pieces. Face part11 with each of the plurality of pole pieces 12 and 13 are as a unitarystructure contoured from the rectangular configuration shown in FIG. 3to a configuration shown by dotted line 26 such that the head assemblyappears as in FIG. 4 in its final form. The top of each of nonmagneticgaps 23 is thus located as shown in FIG. 3 at the intersection of dottedline 26 and the line defining gap region 14. As the depth of gap 23 canbe extremely small, on the order of one-half to several mils, the faceassembly is provided with a wedge shaped reinforcing member 27 ofceramic material mounted as best illustrated in FIG. 3 in mateddisposition within V-shaped groove 24. As described more fullyhereinafter, member 27 may be either epoxied or glass bonded to theinterior walls of groove 24.

Portions of 11a and 11b of the face part are held in an assembledcondition prior to mounting on core housing 16 by means of a pair ofceramic or ferrite tiebars 25. Each tie-bar is a single integral membermounted and epoxy bonded within face part 11 to extend between andsecure together portions lla and 11b. The material for tie-bars 25,either ceramic or ferrite, is selected to have physical propertiessimilar or identical to ceramic face part 11 so as to again eliminateundesirable stress conditions due to differential thermal expansion ofthe various components.

A planar surface 28 of face part 11 is mounted on a corresponding planarsurface 29 of core housing 16 such that the faces of pole pieces 12 and13 flush with surface 28 engage surface portions of one of core membersl7 flush with surface 29 completing a magnetic circuit (as indicated bydotted line 31) traversing gap 23. In order to secure part 11 to housing16, an epoxy bond 32 is provided between the otherwise exposed face 33of wedge member 27 and a previously potted region of surface 29 of thecore housing. For this purpose, it is noted that member 27 isdimensioned so that lower face 33 is recessed in V-groove 24 relative toplanar surface 28.

It has been found that for certain high frequency applications adequateinterchannel shielding can be provided by arranging a plurality ofshields 34, in this instance of laminated soft magnetic metal such asmumetal, between each adjacent pair of core members 17 as illustratedwhere such shields extend generally flush with surface 29 of housing 16.While shields 34 are thus carried wholly within core housing 16 and donot extend into face part 11, shielding has been found sufficient asinterchannel coupling, particularly at higher frequencies, is mostcritical in the regions surrounding the coils of core members 17. Inother transducer applications, it is necessary to extend the shieldinginto face part 11 as illustrated in the embodiment of FIGS. 6A- 6].

Core members 17 with coils 18 mounted thereon and shields 34 are securedin place along with a pair of connector strips 36 by a thin film ofadhesive material between the strips and the side walls of the housing.In the preferred head assembly, core housing 16 is formed of a materialhaving a coefficient of thermal expansion in the same range as that offace part 11, and is in this instance of the identical ceramic materialas the face part.

In accordance with the method of the present invention illustrated inFIGS. 5A-5L, face part 11 is formed from a rectangular block 41 ofceramic material in which a planar face 42 thereof is provided with aplurality of spaced, parallel, rectangular cross section slots 43. Slots43 and other machining operations performed on the ceramic and ferritematerials are carried out using diamond cutters. Thereupon, a pluralityof elongate rectangular magnetic ferrite slugs or pieces, such asferrite piece 44 of FIG. 5B, are formed for mated insertion into each ofslots 43 as best shown by FIG. 5C. Moreover, each of ferrite pieces 44is glass bonded to the walls of slots 43. This glass bonding operationcan be effected in accordance with well known techniques and materials.The process used in this instance provides for initially disposingferrite pieces 44 within slotted block 41 with the various partsdimensioned so as to leave a slight spacing between the exterior of theferrite pieces and the interior walls of slots 43. Thereupon, powderedglass is piled on top of face 42 and the entire assembly is heated at675 C for one hour (such that the glass melt flows by gravity andcapillary attraction into the wall interspaces) and cooled down at amaximum rate of I.5 C per minute. The glass material used in thisprocess is commercially available as Corning Glass No. 7570. It has beenfound that a preferred ceramic material for block 41 may be obtainedfrom Minnesota Mining and Manufacturing Company or G.E. under thegeneric name Forsterite, a material having the composition 2Mg0'SiO-Preferably, the magnetic pieces 44 are formed from a hot pressedferrite, discussed more fully herein, and the ceramic Forsterite, theglass used for bonding, and the ferrite are all selected to havecoefficients of thermal expansion within the same range. i.e., within 20percent of one another.

Having mounted the various ferrite pieces within block 41, face 42 islapped planar and precisely square with the remaining sides of theblock. Also at this time, block 41 is formed with tie-bar slots 46 whichwill later serve with tie-bars 25 to secure the two half block portionstogether. Slots 46 are preferably disposed parallel to pieces 44 andadjacent opposite ends of the block. The block assembly is now out atleast once along a plane normal to face 42 as best illustrated in FIG.5D and at right angles to ferrite pieces 44 so as to form a plurality ofsevered block portions, in this instance consisting of two equal halfportions 41a and 41b. By starting with a single block 41 of ceramicmaterial and cutting it into a plurality of portions, the ferrite polepieces are assured of proper lateral registration in the ultimate headassembly.

With reference now to FIG. 5E, each block portion 41a and 41b isprovided with bevelled faces 47a and 47b. In particular, faces 47a and47b are cut at approximately 45 along a longitudinal edge of the severedblock portions intercepting each ferrite piece 44 as illustrated toleave strip portions of original planar face 42 as surfaces 42a and 42b.The half blocks are now lapped along faces 47a and 47b, surfaces 42a and42b, and surfaces 48a and 48b to insure proper alignment of the ferritepole pieces carried by portions 41a and 41b when they are mountedtogether as described herein.

Having completed these steps, one or both of surfaces 42a and 42b of thehalf block portions are disposed to receive a deposited layer ofnonmagnetic gap forming material, such as layer 49. As described herein,surfaces 42a and 42b are eventually arranged to abut at layer 49 andthus define nonmagnetic gap region 14. Gap forming layer 49 may beprovided by a number of known processes and materials, such as by metalor glass shim, vacuum evaporation or sputtering processes. In thisinstance, it has been found that a preferred gap definition may beobtained by sputtering a layer of Aluminum Oxide (AL O to a desiredthickness onto one or both of surfaces 42a and 42b. The sputteringoperation, due to its high energy characteristics, has been found toform an exceedingly cohesive layer, both internally of the layermaterial itself and in terms of its adhesion to the ferrite and ceramicsurfaces. Typically, the nonmagnetic gap may have a length(corresponding to the thickness of layer 49) ranging from I or 2 pinches on up to 150 p. inches.

At this point, half block portions 41a and 41b are each rotated by 90about their longitudinal axes, such that faces 42a and 42b meet inregistration separated only by layer 49. In this manner, the surfaces offerrite pieces 44 exposed at faces 42a and 42b form the pole faces ofthe magnetic transducer in abuttment at nonmagnetic gap region 14. Theassembly of these two half block portions is best shown in FIG. 5F wherethe parts after assembly have been turned up-side-down. Bevelled faces47a and 47b form, as shown in FIG. 5F, the V-shaped groove 24 discussedabove in connection with FIG. 3. The assembly procedure should be suchthat gap region 14 along the apex of groove 24 is precisely definedwhich in turn requires that the boundary edges 51a and 5112 between face47a and face 42a and between face 47b and face 42b respectively aresubstantially coextensive. This criterion is important in view of alater step in which most of the face part material defining gap region14 is cutaway, as shown in FIG.

3, leaving only a small amount of material to define nonmagnetic gaps23. If the block portions are not precisely positioned with respect toeach other for the entire length thereof, then it will be impossible toattain a uniform depth for gap 23 for all of the pole pieces 12 and 13.

Block portions 41a and 41b are secured together in their assembledrelation by the pair of tie-bars mounted within the tie-bar slots, suchas shown by tie-bar 25 within slot 46, retaining the block halves atopposite ends thereof. In addition, the assembly is strengthened,particularly at gap region 14, by a wedge shaped reinforcing member 53,shown in FIG. 5G, which is shaped and dimensioned to fit in matedrecessed disposition within V-groove 24 of the assembly. In thisinstance, wedge member 53 is of the same Forsterite ceramic material asblock 41 while tie-bars 25 in this instance are of the same ferritematerial as pieces 44. However, member 53 may be provided from anonmagnetic ferrite material and tie-bars 25 may be formed from aceramic material such as Forsterite. In any event, the material chosenfor these parts should have a coefficient of thermal expansion withinthe same range, i.e., within 20 percent, as that of the block 41material and the ferrite material used for pieces 44. For the presentembodiment, wedge member 53 and tie-bars 25 are epoxied into place,using a suitable epoxy such as Able Stick 410-3, available from AbleStick Laboratories, Inc. It is anticipated, however, that these partsmay be glass bonded in place by using a two step glass bonding operationwhereby a first glass material having a relatively high meltingtemperature is employed for bonding ferrite pieces 44 into place andthereafter a second glass material having a lower melting temperature isprovided for securing wedge 53 and tie-bars 25. While it is noted thatan epoxy is suitable for securing member 53 and bars 25 in place, it isnot satisfactory for the pole pieces 12 and 13 which require the hightemperature strength, stability and hardness afforded by the glass bond.

The assembly resulting from these method steps, after precontouring, isillustrated in FIG. 51 and will henceforth be identified in accordancewith FIGS. 1-4, as face part 11 consisting of two half portions 11a and11b. Similarly, ferrite pieces 44 have now become pole pieces 12 and 13as referred to above.

With reference to FIG. SJ, core housing 16 is formed from a set of twoside pieces 56 and 57, preferably of the same ceramic material used forblock 41, and a pair of end members, one of which is shown as member 58,wherein the side pieces are secured to the end members by epoxy bonding.Prior to assembling core housing 16, side pieces 56 and 57 are eachformed with a plurality of slots 61 and 62 for core members 17 andshields 34 respectively. After such assembly, core members 17 andshields 34 are inserted in place and the part of the housing adjacentsurface 29 is potted with a suitable compound such as again Able Stick410-3 and thereafter the portion of the housing carrying connectors 36is potted, in this latter case using a rubbery potting compound RTV(8111 and curing agent NUQCURE 28 available from General Electric).

In the presently described and preferred embodiment and as mentionedabove, shields 34 do not extend into face part 11 and thus offershielding only in the region of coils 18 carried by core members .17.This has been found adequate where high frequency use is intended andthe low frequency specifications are not too stringent. As the shieldsin this instance do not appear at the face of face part 11, it ispossible to use laminated soft magnetic metals such as permalloy ormumetal for these parts. An alternative preferred embodiment in whichshields are disposed in the face part is shown in FIGS. 6A-6J.

As mentioned above, core members 17 are preferably of a ferrite materialbecause of the preferred low loss characteristics thereof. particularlyat high frequencies.

After lapping surface 29 of housing 16 and surface 28 of face part 11,the latter is mounted onto the housing as shown in FIG. K with thevarious pole pieces 12 and 13 offace part 11 in registration with coremembers 17. For the head assembly to operate successfully, it isimportant that the surfaces of pole pieces 12 and 13 exposed at planarsurface 28 be in substantial contact with the pole surfaces associatedwith branches 21 and 22 of core members 17 at surface 29 of the corehousing. For this reason, the lapping of surfaces 28 and 29 should becarried out to within two light bands or approximately microinchesflatness.

With face part 11 arranged as shown in FIG. 5K on top of housing 16, thetwo parts of the head assembly are secured together by an epoxy bond 32as mentioned in connection with FIG. 3. A suitable epoxy is again AbleStick 410-3. In particular, the epoxy material is pressed into an openspace region defined on the one hand by \/-groove 24 and wedge member 53of face part 11 and on the other hand by surface 29 of housing 16. Thisfree space is best illustrated in FIG. 5H. In order to provide access tothe opening from either end of the head assembly, each of tie-bars isformed with a notched-out portion 63 best illustrated in FIG. 5H so thatthe liquid epoxy material can be forced under pressure into this regionfrom one end of the assembly as indicated by arrow 64 in FIG. 5K.Suitable pressure is applied until the epoxy flows out the opposite endof the window. insuring complete filling of the space. After curing,face part 11 is thus bonded to the core housing by only a strip of epoxymaterial underlying groove 24 and member 27. As described and claimed ina US. application for Patent Ser. No. 156,802 entitled MAGNETIC HEADWITH DE- MOUNTABLE FACE PART ASSEMBLY. filed on June 25, 1971 by TonyAntoon Mlinaric. now US. Pat. No. 3.761.64l issued Sept. 25, 1973, thisfeature has the advantage of permitting subsequent removal of face part11, should it prove initially defective or thereafter become defectivein operation. merely by grinding away the face part until surface 29 ofcore housing 16 is reached. Thereupon surface 29 may be relapped and anew face part 11 installed.

In FIG. 5L, the final head assembly is shown in which face part 11 hasbeen contoured to provide a flat region or radius depending upon thedesired shape at the head-to-tape interface 66 with the remainingportions of the front surface of face part 11 sloping away from region66. The contour for the head assembly in its final form is best shown inFIG. 3 by dotted line 26. A typical depth dimension for gaps 23 afterfinal contouring is l-3 thousandths of an inch.

While it has been stated that the material for pole pieces 12 and I3 isa magnetic ferrite. the best results have been obtained with a ferritematerial processed in particular fashion, namely by hot pressing. Whileother ferrites such as those grown from a single crystal aresatisfactory for some'lower performance equipment, the hot pressedferrites, due to their greater internal strength coupled with a highdensity (low porosity). offer the more desirable characteristics for ahigh performance transducer assembly. In general, hot pressed ferritesare formed by starting with a granular form of Maganese or Nickelferrite material and compressing and molding the starting substanceunder high pressure and temperature. The fine grain structure of theresulting material is extremely hard, and although like all ferritessomewhat brittle, it can be cut into the shape of ferrite pieces 44 andbonded in slotted block 41 as described above for forming pole pieces 12and 13.

With reference to FIGS. 6A-6J, an alternative preferred embodiment ofthe invention is shown in which the face part is provided with magneticferrite shields separating each set of pole pieces. For simplification.FIGS. 6A-6.l use the reference numerals of corresponding parts of FIGS.l-5 with a prime added. Thus. FIG. 6.] illustrating the alternativeembodiment of the invention in final form, shows a head assembly with aface part 11' carrying a plurality of pole pieces 12' and 13 forming theplurality of magnetic tracks. Interspaced between these tracks is aplurality of magnetic ferrite shield parts 71. While the previousembodiment of the invention is suitable for applications in which thelow frequency cross talk specifications are not critical, otherapplications require intertrack shielding in the face part itself wherethe shields extend to the head-totape interface. In providing for suchshields in accordance with the present invention, the initial processsteps are the same as described above in connection with FIGS. 5A and5B.

In FIG. 6A, ceramic block 41 is formed with a plurality of shield slots72. similar to tie-bar slots 46' but arranged in between each pair ofadjacent ferrite pieces 44 as illustrated. As in the case of theembodiment of FIG. 5D. block 41' is now cut into at least two portions41a and 41!) as shown in FIG. 6B. and the block portions are processedas described above in connection with FIG. 5E with the result of thesesteps being illustrated by FIG. 6C. The block portions 41a and 41b arenow rotated and arranged as shown in FIG. 6D so that shield slots 72 andtie-bar slots 46 are oriented to receive the respective shield andtie-bar parts.

In FIG. 6F, shield parts 71, which are of a magnetic ferrite material,are mounted within the provided slots 72 while at the same time tie-bars25' are installed within slots 46'. In contrast to the integral wedgeshaped member 53 for the above described assembly, member 53' in thisinstance is sliced into a plurality of segments as shown in FIG. 6E,with each segment being dimensioned so as to fit in the space betweenadjacent shield parts 71 or a shield part and one of tie-bars 25.

Thereupon, shield parts 71, tie-bars 25' and wedge segments 53' areepoxy bonded into place. As an alternative procedure. a two temperatureglass bonding process may be employed with the higher temperature glassused for bonding the ferrite pieces 44' in place and the lowertemperature glass employed for bonding the shield parts, tie-bars andwedge segments.

The portions of shield parts 71 lying within V-shaped groove 24' areformed with notches 73 similar to notches 63 so as to permit the flow ofepoxy in the V- groove region for bonding face part 11 to housing 16 asdescribed herein in connection with FIG. 61. Face part 11' is nowcompleted by precontouring the head face, the result of which is shownin FIG. 6G, and lapping surface 28 for mating with the surface 29 ofcore housing 16.

In FIG. 6H, core housing 16 is fabricated in the same manner asdescribed for the previous embodiment. As in the head assembly of FIGS.1-5, shields 34' may be formed of either a magnetic ferrite material orof a laminated soft magnetic metal, such as mumetal. In this instance,the fabrication process should insure that shields 34' are flush withlapped surface 29 of the core housing, not recessed, so as to meet withand engage shield parts 71 of the face part in a manner similar to theengagement of the pole tips 12 and 13' with magnetic cores 17'. In thismanner, when face part 11 is mounted and secured to the core housing asshown in FIG. 6l, shield parts 71 are magnetically integral withcorresponding shields 34' in housing 16'.

Face part 11 is bonded to housing 16' in the same manner as describedfor the previous embodiment, by forcing a liquid epoxy into the windowspace defined by V-groove 24' and core housing surface 29 as indicatedby arrow 64 in FIG. 61. The completed head is shown by FIG. 6] aftersuitable contouring of the face.

What is claimed is:

l. A multichannel magnetic head assembly comprising a head face part ofnonglassy refractory ceramic material, having a hardness comparable tothat of fer rite materials, said face part having a plurality of spacedparallel slots, a plurality of separate ferrite pole pieces mounted andglass bonded in said slots, said face part being formed of two identicalhalf portions, a layer of nonmagnetic material deposited on onecorresponding side of each said identical half portion, said halfportions being assembled with said corresponding sides in abuttingrelation along a gap line provided by said layer of non-magneticmaterial separating said face part portions and associated pole pieces,a core housing having a surface receiving said face part in matedsecured engagement, and a plurality of magnetic core members having coilmeans mounted in said core housing to register with and engage portionsof said pole pieces thereby forming a plurality of magnetic transducers.

2. The head assembly as defined in claim 1 further comprising aplurality of intercore magnetic shields mounted in said core housingbetween each adjacent pair of core members, said shield members notextending beyond said core housing.

3. The head assembly as defined in claim I further comprising, aplurality of intercore magnetic shields mounted in said core housingbetween each adjacent pair of core members, and a correspondingplurality of shield parts mounted in said face part portions betweeneach adjacent pair of associated pole pieces, said shield parts beingarranged to engage the corresponding shields carried by said corehousing when said face part is in mated secured engagement therewith.

4. The head assembly of claim 1 further defined by said ceramic facepart and ferrite pole pieces and the glass bond therebetween all havingcoefficients of thermal expansion selected to fall within the samerange.

5. The head assembly as defined in claim 1, said face part having atleast one tie-bar slot intercepting both half portions and a tie-barmounted and bonded in said slot to secure said housing portions inassembled relation.

6. The head assembly as defined in claim 1, the engaged surfaces of saidface part and core housing being generally planar and such face partsurface having a V- shaped groove underlying said gap line, a wedgeshaped member of ceramic material matingly disposed and bonded in saidgroove to reinforce said face part por' tions at said nonmagnetic gap.

1. A multichannel magnetic head assembly comprising a head face part ofnonglassy refractory ceramic material, having a hardness comparable tothat of ferrite materials, said face part having a plurality of spacedparallel slots, a plurality of separate ferrite pole pieces mounted andglass bonded in said slots, said face part being formed of two identicalhalf portions, a layer of nonmagnetic material deposited on onecorresponding side of each said identical half portion, said halfportions being assembled with said corresponding sides in abuttingrelation along a gap line provided by said layer of non-magneticmaterial separating said face part portions and associated pole pieces,a core housing having a surface receiving said face part in matedsecured engagement, and a plurality of magnetic core members having coilmeans mounted in said core housing to register with and engage portionsof said pole pieces thereby forming a plurality of magnetic transducers.2. The head assembly as defined in claim 1 further comprising aplurality of intercore magnetic shields mounted in said core housingbetween each adjacent pair of core members, said shield members notextending beyond said core housing.
 3. The head assembly as defined inclaim 1 further comprising, a plurality of intercore magnetic shieldsmounted in said core housing between each adjacent pair of core members,and a corresponding plurality of shield parts mounted in said face partportions between each adjacent pair of associated pole pieces, saidshield parts being arranged to engage the corresponding shields carriedby said core housing when said face part is in mated secured engagementtherewith.
 4. The head assembly of claim 1 further defined by saidceramic face part and ferrite pole pieces and the glass bondtherebetween all having coefficients of thermal expansion selected tofall within the same range.
 5. The head assembly as defined in claim 1,said face part having at least one tie-bar slot intercepting both halfportions and a tie-bar mounted and bonded in said slot to secure saidhousing portions in assembled relation.
 6. The head assembly as definedin claim 1, the engaged surfaces of said face part and core housingbeing generally planar and such face part surface having a V-shapedgroove underlying said gap line, a wedge shaped member of ceramicmaterial matingly disposed and bonded in said groove to reinforce saidface part portions at said nonmagnetic gap.